Conventionally, composite reverse osmosis membranes have been known as reverse osmosis membranes which differ in structure from asymmetric reverse osmosis membranes. Such composite reverse osmosis membranes are produced by forming active thin films having selective separativeness on microporous supports.
Applications disclose membranes comprising polyamide obtainable by interfacial polymerization between polyfunctional aromatic amines and polyfunctional aromatic acid halides formed on supports. The examples of such applications are, Unexamined Published Japanese Patent Application (Tokkai-Sho) 55-147106, Tokkai-Sho 62-121603, Tokkai-Sho 63-218208, and Unexamined Published Japanese Patent Application (Tokkai-Hei) 2-187135.
Inclusion of hydrophilic groups in membranes to improve water permeability is disclosed by, for example, Tokkai-Sho 57-119928, Tokkai-Sho 59-179103, and Tokkai-Sho 62-53703. It is also disclosed that water permeability is further improved by a treatment with an aqueous alkaline solution after including hydrophilic groups in the membrane (see Tokkai-Sho 63-7807 etc.).
Highly hydrophilic polyimide-based reverse osmosis membranes, which include polyvinyl alcohols, are also disclosed in Examined Published Japanese Patent Application (Tokko-Sho) 59-27202, Tokko-Sho 61-17521, Examined Published Japanese Patent Application (Tokko-Hei) 2-32009, Tokkai-Sho 63-17521 and the like.
However, these membranes have a salt rejection factor greater than 90%, and the permeable water quantity is no more than 0.2 m3/m2·d·kg/cm2. This indicates that the permeable water quantity level is as low as 0.4 m3/m2·d·kg/cm2 at a pressure level of city water (2 kg/cm2). When such membranes are used, the membrane area should be enlarged or pumps should be employed to raise pressure to obtain a proper permeable water quantity. Because these materials reject such a high percentage of salt, salt concentration in the concentrated water is increased at an operation with a high recovery rate, so that insoluble ingredients such as CaCO3 and SiO2 are deposited on the membrane surface and cause troubles such as a decrease in permeable water quantity.
In general, the above-mentioned composite reverse osmosis membranes cannot function sufficiently until they are operated under a pressure of at least 10 kg/cm2. This raises costs for facilities and operation. To solve this problem, more improved composite reverse osmosis membranes are required. An ideal membrane would remove toxic substances and maintain economical permeation water level at low pressures. Improvement in the recovery rate (=permeable water quantity/supplied water quantity) can reduce the quantity of supplied water and of condensed water for obtaining a predetermined quantity of permeable water. This results in a smaller apparatus as well as lowering costs for running and treatment of waste water. However, a high recovery rate causes the concentration of insoluble ingredients to rise, causing clogging of the surface. A membrane, which retards deposition of insoluble ingredients even in a high recovery rate of no less than 80%, is required.